1. Field of the Invention
The present invention relates to lift and hoist mechanisms, more particularly, to a lift assembly that can be employed for raising and lowering a load in theatrical and staging environments, wherein the lift assembly is a modular self contained unit that can be readily installed in a wide variety of building configurations.
2. Description of Related Art
Performance venues such as theaters, arenas, concert halls, auditoriums, schools, clubs, convention centers and television studios employ battens or trusses to suspend lighting, scenery, drapery and other equipment which is moved relative to a stage or floor. These battens usually include pipe or joined pipe sections that form a desired length of the batten. The battens can be 50 feet or more in length. To support heavy loads or where suspension points are spaced 15–30 feet apart, the battens may be fabricated in either ladder, triangular or box truss configurations.
Battens often need to be lowered for exchanging and servicing the suspended equipment. To reduce the power necessary to raise and lower the battens, the battens are often counterweighted. The counterweights reduce the effective weight of the battens and any associated loads.
A typical counterweight system represents a significant cost. The creation of T-bar wall 70 feet to 80 feet in height and 30 feet deep may require over three weeks. Even after installation of the T-bar wall, head block beams, loading bridges, index lights and hoist systems must be integrated. Therefore, a substantial cost is incurred in the mere installation of a counterweight system. The total installation time may range from 6 to 12 weeks.
A number of elevating or hoisting systems are available for supporting, raising and lowering battens. One of the most common and least expensive batten elevating systems is a counterweighted carriage which includes a moveable counterweight for counterbalancing the batten and equipment supported on the batten.
Another common elevating or hoisting system employs a winch to raise or lower the battens. Usually hand or electric operated winches are used to raise or lower the battens. Occasionally in expensive operations, a hydraulic or pneumatic motorized winch or cylinder device is used to raise and lower the batten.
Many elevating systems have one or more locking devices and at least one form of overload limiting device. In a counterweight system, a locking device may include a hand operated rope that is attached to one end of the top of the counterweight arbor (carrying device) and then run over a head block, down to the stage, through a hand rope block for locking the counterweight in place, and then around a floor block and back up to the bottom of the counterweight arbor. The hand rope lock locks the rope when either the load connected to the batten or the counterweight loads are being changed and rebalanced and locks the loads when not moving.
In a sandbag counterweight system, the locking device is merely a rope tied off to a stage mounted pin rail, while the overload limit is regulated by the size of the sandbag. In this rigging design, however, a number of additional bags can be added to the set of rope lines, and thereby exceed the safe limit of suspension ropes and defeat the overload-limiting feature.
Hand operated winches will occasionally free run when heavily loaded and will then dangerously drop the suspended load. Other types of hand winches use a ratchet lock, but again these winches are also susceptible to free running when they are heavily loaded and hand operated.
Therefore, the need exists for a lift assembly that can replace traditional counterweight systems. The need further exists for a lift assembly that can be readily installed into a variety of building configurations and layouts. A need further exists for a lift assembly having a modular construction to facilitate configuration to any of a variety of installations. A need also exists for a lift assembly that can maintain a predetermined fleet angle during raising or lowering of a load.